In the manufacture of high purity products, such as silicon wafers, intended for semiconductor substrates or in the photolithography steps of manufacture of semiconductors, it is necessary to maintain a high degree of cleanliness. The products themselves must be clean, the atmospheres surrounding them throughout the manufacturing process must be clean, and the steps and equipment used in the manufacture must not impair cleanliness. It is well known that with the minute sizes of circuitry and components incorporated into semiconductor chips, even extremely small contaminant particles when deposited on chip surfaces are destructive to the chips. It is common for loss rates of wafers and chips during manufacturing to be a significant portion of the total production due to system contamination.
Manufacturers of wafers and chips have been engaged in extensive and continual efforts to improve on the cleanliness of their fabrication facilities (“fabs”) including efforts to have manufacturing and process materials and gases of high purity. Such efforts have been generally successful in the past, in that gases with purities defined by contaminant levels in the parts per million (ppm) and even into the parts per billion (ppb) ranges have been achieved. Generally improvements in process system cleanliness have paralleled increases in the component density of chips and reductions in the size of chip components and circuitry.
However, the ability of the prior art to achieve such parallel improvements in gases has more recently been severely taxed as the size of chip components has continued to decrease and component density has continued to increase. With the movement to 198 nm and 157 nm semiconductor technologies, the ability of the products to tolerate contamination has substantially decreased, and process gases that previously were of adequate purities are no longer suitable. Scale-up techniques that previously achieved adequate improvements in the purity of such gases have been found to be ineffective in these “ultra high purity” (UHP) systems in which the lower nm level technologies are produced. Further, at the lower IC dimensions materials that were previously considered minor contaminants have been found to act as major contaminants, and the prior art gases have been found to be ineffective in removing such contaminant materials.
Ultrahigh purity products and process tools are susceptible to airborne molecular contaminants (AMCs) that can reduce product quality and yield. AMCs generally include, but are not limited to SOx, NOx, siloxanes, organophosphorus compounds, ammonia, moisture, oxygen and hydrocarbons (>4 carbons). For purposes of the present invention, oxygen and moisture are not considered to be AMCs.
In the production of wafers for the semiconductor industry, there are three major sources of contamination, wafer storage containers (for example, Front-Opening Unified Pods or FOUPs) themselves; clean room air that enters the container as the wafers are moved between tools and the wafers themselves that may leech contaminants during the various manufacturing processes. Methods have been developed to sufficiently reduce water and oxygen contamination in the manufacturing process. Additionally, methods have been developed for the removal of reaction products of the wafer with water and oxygen (e.g., silicon oxides) that can form on the surface of the wafers. However, technologies have not developed for the efficient removal of a number of airborne contaminants and their resulting reaction products on wafers.
Various contaminants have different effects. For example, in photolithography simple hydrocarbons can condense on the lens assembly and result in transmission loss. Heavy hydrocarbons and significant concentrations of light hydrocarbons irreversibly deposit on optical surfaces and become graphitized by ultraviolet (UV) exposure. In a similar manner, silicon-containing organics, e.g., siloxanes, react under UV irradiation to produce SiO2 crystallites that refract and absorb the incident light. Other AMCs, e.g., NOx and SOXx, typically wherein 0<x≦3, cause optical hazing. Basic AMCs, e.g., amines, quench the photoacids, in addition to causing optical hazing. In the context of photolithography, oxygen and water can be detrimental to the production process and are typically considered to be AMCs in the prior art. Recently, its has been reported by Veillerot et al., (Solid State Phenomena Vol. 92, 2003, pp. 105-108) that atmospheric hydrocarbon contamination has a detrimental impact on 4.5 nm gate oxide integrity when wafers are stored in a continuous flow of purge gas between gas oxide and polysilicon deposition steps.
Approaches being tried to reduce this contamination include large-scale chemical filtration of the cleanroom air, moving from open to closed cassettes, and nitrogen purging of wafers during storage and transport. Nitrogen purging of UHP components such as valves and gas delivery piping, has been practiced for many years, and can be effective in removing oxygen and water. However, large scale use of nitrogen for purging large volume IC process equipment and large numbers of cassettes can be expensive and present a serious asphyxiation hazard. Additionally, it is suspected that nitrogen purging of hydrocarbon contaminated surfaces is not completely effective in removing the hydrocarbons.
Methods for analysis of contaminants in gas streams are well known. FIG. 1 (Prior Art) is a schematic flow diagram of a double dilution system 100 coupled to a gas chromatograph gas analysis system 120. The double dilution system 100 comprising mass flow controllers 106, 108, 110, and 112 enables the precise dilution of a gas standard 114 with a carrier gas 102 over a range of six orders of magnitude (106). Commonly available gas standards in the part per million range can be effectively diluted to the part per trillion (ppt) range with system 100. The dilution system 100 can be coupled to a gas chromatograph system 120 for the purposes of calibrating the response of the chromatograph 126, by connecting the output 116 of the dilution system to the input 122 of the chromatographic gas analysis system 120. A cold trap 124 accumulates condensed hydrocarbons in the sample, prior to injection into the gas chromatograph 126. In this manner, the effective sensitivity of the chromatograph can be increased and ppt level hydrocarbon concentrations reliably measured.
FIG. 2 (Prior Art) is a calibration graph 200 of signal response area 204 versus sample hydrocarbon concentration 202 for various hydrocarbon molecules including benzene 206, toluene 208, ethyl-benzene 210, meta,para-xylene 212, ortho-xylene 214, a second toluene 216, for the analysis system 120 coupled to dilution system 100. The data 220 show a linear response relationship between the peak area 204 and concentration 202 over almost six orders of magnitude, with a minimum sensitivity of 1 ppt
FIG. 3 (Prior Art) is a graph 300 of time 302 versus gas chromatograph 126 detector signal 304 for a sample containing 1 ppt each of benzene, toluene, ethyl-benzene, and xylene. Here it can be seen that 1 ppt level concentrations for each of the hydrocarbons in the mixture result in clearly distinguished peaks for benzene 306, toluene 308, ethyl-benzene 310, and xylene 312.